The search for, and production of, oil through the ocean floor is often accomplished using a floating marine vessel subject to considerable vertical motion due to the heaving of the waves. During storm conditions when the sea is abnormally high, operations, particularly drilling operations, must be curtailed, running the risk of the drill pipe sticking in the hole or the risk of pulling the string from the well in inclement weather to remove the bit from the bottom. Often, damage results in the bore hole at great expense. Other precise operations occur during operations, such as, for example, setting a blowout preventer around the wellhead on the ocean floor, logging the wellbore, perforating casing into a producing zone, setting a packer and the like.
During any such operation from the platform of a vessel moored at sea, compensation must be made for the changes in level due to wave action. Hydropneumatic cable tensioners are used in the mooring. Examples of such devices are described in U.S. Pat. Nos. 3,314,657; 4,540,159; and 4,638,978. In addition, motion compensators are found to be useful with almost any load carrying device used at sea, such as, for example, a crane as described in U.S. Pat. Nos. 3,311,351 or 3,718,316. However, by far the most pressing use has been to compensate the motion around the drill string itself, either at the crown block (U.S. Pat. No. 3,791,628), an in line compensator between the hook and the traveling block (U.S. Pat. Nos. 3,403,728 or 3,841,607) and on the traveling block itself (U.S. Pat. Nos. 3,912,227 and 3,718,316; 3,804,183, for example). The disclosures of the foregoing patents and principles discussed therein are all well known and incorporated herein by reference for all purposes to set forth the state of the art against which this invention is made. The most common drill string compensators are carried on the traveling block. In one system, typically the disclosure of U.S. Pat. No. 3,804,183, the piston rod is under compression forces from the hook upon which the drill string is hung. In another system it is under tension as can be seen in U.S. Pat. No. 3,718,316.
It is common among all such motion compensators to employ a moving piston and rod system separating two dissimilar fluids as shock absorbers, acting easily as a spring, usually compressed air and a hydraulic fluid employing compressed air reservoirs and an accumulator vessel, to transmit the fluids to compensate for pressure changes brought about by the heaving of the sea surface. Just as the elements of the various assemblies are common and well known, also known is the degree of accuracy with which a load on the hook can be held in one place and the large response time between a change in the level of the sea and the application of pressure from the accumulator into the cylinder and piston system of the motion compensating apparatus to return the system to its working conditions.
In most cases, the accumulator sits on the drill floor with fluid lines running some distance to the motion compensating system, either at the crown block or at the traveling block. Indeed, in some instances, such as the hydropneumatic cable tensioners described in U.S. Pat. Nos. 4,540,159 and 4,638,978, the accumulator is in an annulus of the body of the compensating system. Even then response time losses occur and load variations are left unresolved. The load variations and time lapse for response arise from a number of factors; such as the large volume of air in the commonly used air bank, piston friction losses within the cylinders and pressure drop in the fluid (oil) and air conduits. In drilling systems, in order to compensate for wide weight variations, very strong drill pipe is used necessitating larger drill bits, larger casing and more difficult, and expensive, drilling operations. Heavier, drill strings were required to avoid breakage due to the heaving jerks on the string or inaccurate bit control due to heavy weight on the bit causing a bend in the drill string. In attempts to use lighter drill strings and smaller bits is so-called slimhole drilling, the need for closer control of weight variation on the bit is more critical.
Attempts have been made to devise active heave compensating devices for marine use which would aid the accumulator in compensating for pressure changes in the heave compensation system. One such system is described in U.S. Pat. No. 3,946,559. The device, while an improvement, still leaves much to be desired and the overall problem has remained unsolved until now.
It is therefore an object of this invention to provide a system which will compensate for the heave of waves on a marine vessel with reduced response time and variations in weight control on a drill bit in a drilling system.
It is a further object of this invention to provide a system for compensating for the heave of waves on marine vessels which will reduce the effects of bad weather on drilling operations, reducing the necessity to remove the bit from the borehole being drilled in the subsurface floor.
It is a further object of this invention to provide an improved motion compensation, or weight control, system which can be added to an existing passively operated piston and cylinder compensating system on drill string compensators, cable tensioners, cranes and the like on marine vessels.
It is a further object of this invention to provide a drill string compensation system to improve weight control in compensating apparatus which operates either under tension or under compression when the hook is loaded.
It is a further object of this invention to provide a system for improved weight control by passive heave compensating systems used as drill string compensators, cable tensioners and a load carrying hook on a crane.